Accumulator and manufacturing process thereof

ABSTRACT

Disclosed is an accumulator comprising: a pressure vessel; an elastic bellows in which a compressed gas is sealed, an end of the bellows being fixed to an interior of the pressure vessel; a flow path having an opening communicating with the interior and an exterior of the pressure vessel; a valve connected to a movable end of the bellows to operatively close the opening according to elastic motion of the bellows; and a hydraulic chamber partitioned from a gas chamber formed in an interior of the bellows containing the compressed gas. The valve comprises an upper surface which can cover the opening, and plural circular protrusions which surround the entire circumference of the opening and can closely contact the circumference of the opening.

This is a Division of application Ser. No. 09/569,299 filed May 11,2000. The disclosure of the prior application is hereby incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an accumulator having a bellows, whichis used for pressure accumulation or pulsation absorbing in hydraulicdevices in automobile brake systems and various industrial hydraulicsystems. The present invention further relates to a manufacturingprocess for accumulators such as the above, and more specificallyrelates to a joining method for shells which form a vessel for enclosingoil and gas therein.

2. Background Art

FIG. 5 shows an accumulator used for hydraulic devices in automobilebrake systems and various industrial hydraulic systems. The inside of ahousing 31 acts as a pressure vessel and is partitioned into a gaschamber 33 in which a compressed gas is sealed therein and a hydraulicchamber 34 by a metallic bellows assembly 32. The metallic bellowsassembly 32 comprises an elastic metallic bellows 35, of which one endis fixed to the housing 31 and the free end thereof is equipped with avalve 37. The hydraulic chamber 34 defined in the interior of thehousing 31 and exterior of the metallic bellows 35 is communicated withan external system through an opening 38 a provided in the housing 31and a flow path 38.

In such accumulator, when the hydraulic pressure transmitted from theflow path 38 is lower than the gas pressure sealed in the gas chamber 33and the pressure in the hydraulic chamber 34 coincides with the lowpressure, a differential pressure occurs between the gas chamber 33 andthe hydraulic chamber 34. As a result, the metallic bellows 35 isextended and the valve 37 is thrust and closely contacted to thecircumference of the opening 38 a, thereby closing the opening 38 a, andthe valve 37 therefore self-seals so as to make the pressure of thehydraulic chamber 34 greater that of the gas chamber 33.

The valve 37 may not be able to exhibit sealing properties in closingdue to factors such as aging degradation thereof and jamming of foreignmatter such as dust. When the pressure transmitted from the flow path 38is low, the pressure in the hydraulic chamber 34 also becomes low. As aresult, stress is generated in the metallic bellows 35 due to thedifferential pressure between the hydraulic chamber 34 and the gaschamber 33, so that the durability thereof is lowered.

The end plate of the accumulator must be thick since it is slab-shaped,which results in increased weight of the overall accumulator. Therefore,end plates having semicircular or semi-ellipsoid cross section, whichcan disperse stress, are mainly used.

In accumulators, the stroke of the metallic bellows contained therein isessential for designing the volume of the gas to be sealed therein. Thecylindrical portion of the pressure vessel is effective for pressureaccumulation. In the end plates having semicircular or semi-ellipsoidcross section, the curved portion is a dead space and is generallyemployed merely for containing the liquid. Therefore, it has beendesired to effectively use this dead space.

The accumulator absorbs pulsation by the elastic motion of the metallicbellows. However, the pressure of the pulsation occurring at a pressurelower than that of the sealed gas is lower than the operating pressureof the accumulator, so that the pulsation cannot be absorbed by theabove construction. Heretofore, a special resonance box having afrequency corresponding to the pulsation is provided to absorb thepulsation. This results in large design and increased weight of theaccumulator.

The pressure vessel of the accumulator consists of at least two shellsfor containing the bellows and other necessary parts, and such amanufacturing process is applied so that the bellows and the like areattached to one shell, and another shell is then put over the bellowsand the like and is joined to the other shell. In the conventionaljoining method, the outer surface portion of the joining portion hasbeen welded over the entire circumference by gas welding or tungsteninert gas welding.

However, these welding methods require long operation time, andmass-production efficiency is therefore not good and production cost isrelatively high. Therefore, developments in methods for efficientlyjoining shells have been desired.

SUMMARY OF THE INVENTION

An object of the present invention is to improve reliability ofself-sealing properties of accumulators. Another object of the inventionis to provide an accumulator which can efficiently use the dead spaceformed by the end plate having semicircular or semi-ellipsoid crosssection in the pressure vessel and can absorb pulsation with optionalfrequency without large design and being heavy. A further object of theinvention is to provide a manufacturing process for accumulator, inwhich shells are efficiently joined, manufacturing time is shortened,and manufacturing cost is decreased.

The invention provides an accumulator comprising: a pressure vessel; anelastic bellows in which a compressed gas is sealed, an end of thebellows being fixed to an interior of the pressure vessel; a flow pathhaving an opening communicating with the interior and an exterior of thepressure vessel; a valve connected to a movable end of the bellows tooperatively close the opening according to elastic motion of thebellows; and a hydraulic chamber partitioned from a gas chamber formedin an interior of the bellows containing the compressed gas; wherein thevalve comprises an upper surface which can cover the opening, and pluralcircular protrusions which surround the entire circumference of theopening and can closely contact the circumference of the opening.

Furthermore, the inventors noted that a hydraulic chamber may functionas a resonance box according to the frequency of pulsation, and havemade the invention based on this. The invention provides an accumulatorcomprising: a pressure vessel having an end plate curving convexlyoutward; an elastic bellows having two ends, one of the ends beingconnected to the end plate of the pressure vessel via a plug member andthe other of the ends being closed so as to partition the interior ofthe pressure vessel into a hydraulic chamber communicated with anexterior system and a gas chamber sealing a compressed gas; and aresonance box formed at the plug member in a location of the end plateso as to absorb predetermined pulsation; wherein the plug member isreplaceable.

The invention further provides a manufacturing process for anaccumulator, the process comprising: assembling a buffer member into acylindrical shell so as to partition a interior of the shell into a gaschamber and a hydraulic chamber; and closing the shell; wherein theshell comprises shell portions divided in the direction of an axis ofthe shell; a circular circumferential portion projecting outward isformed over the entire circumference of each joining portion of theshell portions; a circular protrusion projecting in a joining directionis formed at the circular circumferential portion of at least one of theshell portions; the circular protrusions are brought into contact witheach other, or alternatively the circular protrusion is brought intocontact with the circular circumferential portion of another shellportion; the circumferential portions are clamped and pressed by a pairof electrodes; and the electrodes are energized so as to join thejoining portions by electric resistance welding.

According to the manufacturing process for an accumulator in theinvention, the joining portions of the shell portions are directlypressed and clamped by the electrodes. The circumferential portionprojecting outward is formed so that the electrodes come into proximitywith each other. When the circular protrusion is formed in eachcircumferential portion, the circular protrusions are brought intocontact with each other. When the circular protrusion is formed in oneof the circumferential portions, the circular protrusion is brought intocontact with another circumferential portion. The electric resistancewelding through bringing protrusions into contact each other is called“projection welding”, in which the welding is performed over the entirecircumference with instantaneous energization. Therefore, the timerequired for welding the shell portions can be greatly shortenedcompared to the conventional welding method. As a result, massproduction efficiency is improved and manufacturing cost can be reduced.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a side cross section of an accumulator of the first embodimentaccording to the invention.

FIG. 2 is a side cross section showing the operation of the accumulatorof the first embodiment according to the invention.

FIG. 3 is a side cross section of an arrangement of the accumulator ofthe first embodiment according to the invention.

FIG. 4 is a side cross section of the arrangement of the accumulator ofthe first embodiment according to the invention.

FIG. 5 is a side cross section of a conventional accumulator.

FIG. 6 is a side cross section of an accumulator of the secondembodiment according to the invention.

FIG. 7 is a side cross section of the accumulator showing the conditionin which the plug member is replaced.

FIG. 8 is a side cross section of an accumulator of the third embodimentaccording to the invention.

FIGS. 9A and 9B are cross sections showing a process in which a bottomseal and a port of a bellows are welded by projection welding, whereinFIG. 9A shows a status before welding and FIG. 9B shows a status afterwelding.

FIGS. 10A and 10B are cross sections showing a process in which a capshell and a port are welded by projection welding, wherein FIG. 10Ashows a status before welding and FIG. 10B shows a status after welding.

FIGS. 11A and 11B are cross sections showing a process in which a capshell and a plug retainer are welded by projection welding, wherein FIG.11A shows a status before welding and FIG. 11B shows a status afterwelding.

FIG. 12 is a cross section of a joining portion showing a status beforea bottom shell and a cap shell are welded by projection welding.

FIG. 13 is a cross section of an arrangement of the accumulatoraccording to the invention.

DETAILED EXPLANATION OF THE INVENTION

Preferred embodiments of the invention will be explained in detailhereinafter.

FIG. 1 shows a cross section of an accumulator according to the firstembodiment of the invention. Reference numeral 1 is a housing acting asa pressure vessel. The interior of the housing 1 is partitioned by ametallic bellows assembly 2 contained therein into a gas chamber 3 and ahydraulic chamber 4. The metallic bellows assembly 2 comprises ametallic bellows 5 having plural ribs. An end of the metallic bellows 5is fixed to the housing 1 and the metallic bellows 5 is elastic in theaxial direction of the housing 1. The assembly 2 comprises a plate 6fixed to the free end of the metallic bellows 5 and a valve 7 fixed tothe plate 6. The valve 7 is made from elastomeric materials such asrubber. A compressed gas is sealed in the gas chamber 3 in the metallicbellows 5. The hydraulic chamber 4 defined in the housing 1 and in theexterior of the metallic bellows 5 is communicated with an externalsystem through an opening 8 a and a flow path 8 formed in the housing 1.

A bellows guide 10 is fixed at the circumference of the plate 6. Thebellows guide 10 is ring-shaped and slideably fitted into the innersurface of the housing 1. Plural through holes 10 a are formed at theouter circumference of the bellows guide 10 therealong at regularintervals. The bellows guide 10 support the free end of the metallicbellows 5 so that it may not swing in the elastic motion thereof. In theelastic motion of the metallic bellows 5, the hydraulic fluid in thehydraulic chamber 4 passes through the through hole 10 a.

According to the accumulator, when the pressure transmitted through theflow path 8 is lower than that of the gas sealed in the gas chamber 3and the pressure in the hydraulic chamber 4 coincides with the lowpressure, a differential pressure occurs between the gas chamber 3 andthe hydraulic chamber 4. As a result, the metallic bellows 5 is extendedand the valve 7 is thrust and closely contacted with the circumferenceof the opening 8 a, thereby closing the opening 8 a, and the valve 7therefore self-seals so as to make the pressure of the hydraulic chamber4 greater that of the gas chamber 3.

The valve 7 consists of an upper surface portion 7 a which can cover theopening 8 a, a first circular protrusion 7 b surrounding the entirecircumference of the opening 8 a and which closely contacts thecircumference of the opening 8 a, and a second circular protrusion 7 cwhich is formed around the first circular protrusion 7 b and whichclosely contacts the entire circumference of the opening 8 a.

The operation of the above accumulator so constructed will be explainedwith reference to FIGS. 1 and 2 hereinafter. When the hydraulic pressurein the flow path 8 is greater that in the gas chamber 3, as shown inFIG. 2, the metallic bellows 5 is contracted, the valve 7 separates fromthe opening 8 a, the hydraulic chamber 4 and the flow path 8 arecommunicated with each other, and pressure is accumulated according tothe contraction of the metallic bellows 5.

When the hydraulic pressure in the flow path 8 is decreased due tofactors such as pulsation, stopping, and variation of load in a pump(not shown), the metallic bellows 5 is extended to compensate for thehydraulic pressure in the flow path 8. When the hydraulic pressure inthe flow path 8 becomes lower than that in the gas chamber 3, as shownin FIG. 1, the valve 7 is closely contacted with the valve seat 9 andcloses the opening 8 a. The valve 7 has a dual structure consisting ofthe first circular protrusion 7 b and the second circular protrusion 7 cformed therearound. Therefore, there is no concern that the hydraulicpressure in the hydraulic chamber 4 will decrease since one of thecircular protrusions reliably seal the opening 8 a even if the sealingproperties in one of the circular protrusions 7 b and 7 c are decreaseddue to the factors such as aging degradation thereof and jamming offoreign matter.

It should be noted that although the heights and flexibility of thecircular protrusions 7 b and 7 c are the same in the above accumulator,the flexibility of the first circular protrusion 7 b may be greater thanthat of the second circular protrusion 7 c by changing the thicknessand/or material thereof, and the height of the first circular protrusion7 b may be relatively greater than that of the second circularprotrusion 7 c, so that the first circular protrusion 7 b closes theopening 8 a in advance and not simultaneously rather than the secondcircular protrusion 7 c. In such constructions, the surface pressure ofthe first circular protrusion 7 b is greater than that of the secondcircular protrusion 7 c, so that the sealing properties in the usualoperations can be further improved.

FIG. 3 is a drawing similar to FIG. 1, showing an arrangement of thefirst embodiment. In FIG. 3, corresponding numerals as in FIG. 1 areattached to the same elements as in FIG. 1, and detailed explanation ofthe same elements will be omitted. In the structure, the valve 17 is thesame as in the first embodiment in having a dual structure consisting ofan upper surface portion 17 a, a first circular protrusion 17 bprojecting downward from the first circular protrusion 17 b, and asecond circular protrusion 17 c formed around the first circularprotrusion 17 b. However, the length H1 in the projection direction ofthe inner first circular protrusion 17 b is longer than the length H2 inthe projection direction of the second circular protrusion 17 c. Thatis, a difference in level is provided between the both. Furthermore, thevalve seat 19 at the opening 8 a side is also formed with a difference.In this structure, the inner first circular protrusion 17 b essentiallyreceives the pressure, and the outer second circular protrusion 17 cinhibits entry of foreign matter, such as dust, into the first circularprotrusion 17 b, and the sealing properties thereof can be furtherimproved. Other structures and advantages, in which there is no concernthat the hydraulic pressure in the hydraulic chamber 4 will decreasesince one of the circular protrusions reliably seals the opening 8 aeven if the sealing properties in one of circular protrusions 17 b and17 c is decreased due to the factors such as aging degradation thereofand jamming of foreign matter, are the same as in the first embodiment.

FIG. 4 is similar to FIG. 3, showing another arrangement of the firstembodiment. In the structure, the valve 17 is the same as in the firstembodiment in having a dual structure consisting of an upper surfaceportion 27 a, a first circular protrusion 27 b projecting downward fromthe first circular protrusion 27 b, and a second circular protrusion 27c formed around the first circular protrusion 27 b. However, the lengthH3 in the projection direction of the outer second circular protrusion27 c is longer than the length H4 in the projection direction of theinner first circular protrusion 27 b. That is, a reverse difference inlevel is provided between the both. Furthermore, the valve seat 29 atthe opening 8 a side is also formed with a differential corresponding tothe above difference in level. The functions and advantages in thearrangement is the same as in the above arrangement.

It should be noted that although the dual structure consisting of thefirst and second circular protrusions is applied in the arrangements, afurther circular protrusion may be provided around the second protrusionso as to form a triple structure.

The second embodiment of the invention will be explained hereinafter.

FIG. 6 is a cross section of an accumulator according to the invention,and numeral 101 is a housing acting as a pressure vessel. The housing101 has a cylindrical shape with a bottom. The interior of the housing101 is partitioned into a hydraulic chamber 103 and a gas chamber 104 bya metallic bellows assembly 102. The metallic bellows assembly 102comprises a metallic bellows 105 having plural ribs and elastic in theaxial direction of the housing 101, a free end cap 106 provided at thefree end of the metallic bellows 105, and a base end cap 111 provided atthe base end of the metallic bellows 105. The metallic bellows assembly102 further comprises a valve 107 which is made from elastomericmaterials such as rubber and is attached to the free end cap 106 at theinside of the metallic bellows 105. The metallic bellows assembly 102 isfixed to the housing 101 by fixing the base end cap 111 to thebellow-mentioned plug 108.

The hydraulic chamber 103 is communicated with an external systemthrough an opening 109 a, the plug 108 provided to the end plate of thehousing 101, and a flow path 109. In the gas chamber 104 which isdefined in the housing 101 and in the exterior of the metallic bellows105, a compressed gas and suitable amount of an operating fluid foradjusting the spring constant of the metallic bellows 105 in theexpansion and contraction thereof are sealed.

The end plate 101 a, which is removably attached at the base end side ofthe housing 101 by a suitable means such as a screw, has a cross sectionof which the configuration is a semi-ellipsoid in which the proportionof the major axis to the minor axis is 4:1. It should be noted that theend plate 101 a may be fixed to the housing 101 after the shape and thesize of the resonance box 110 are determined. The plug 108 is formedwith a resonance box 110 which is contained in the dead space L formedby the end plate 101 a. The resonance box 110 enables to absorbpulsation occurring in a lower pressure than the operating pressure ofthe accumulator.

A bellows guide 112 is fixed at the circumference of the free end cap106. The bellows guide 112 is ring-shaped and slideably fitted into theinner surface of the housing 101. Plural through holes 112 a are formedat the outer circumference of the bellows guide 112 therealongat_regular intervals. The bellows guide 112 support the free end of themetallic bellows 105 so that it may not swing in the elastic motionthereof. In the elastic motion of the metallic bellows 105, the gas inthe gas chamber 104 passes through the through hole 112 a.

Plural plugs 108 with resonance boxes 110 having various volumes areprepared according to frequencies of pulsation to be absorbed, and arereplaceable according to the system to which the accumulator is to beattached (see FIG. 7).

According to the accumulator, when the hydraulic pressure in the flowpath 109 is greater than that of the gas sealed in the accumulator, themetallic bellows 105 is extended until the pressure in the flow path 109coincides with the pressure in the gas chamber 104, and the valve 107separates from the opening 109 a, so that the hydraulic chamber 103 andthe flow path 109 are communicated with each other, and pressure isaccumulated according to the extension of the metallic bellows 105. Whenthe hydraulic pressure in the flow path 109 is decreased due to thefactors such as pulsation, stopping, and variation of load in a pump(not shown), the metallic bellows 105 is contracted to compensate thehydraulic pressure in the flow path 109. When the hydraulic pressure inthe flow path 109 becomes lower than the gas pressure in theaccumulator, as shown by a virtual line, the valve 107 is thrust to thecircumference of the opening 109 a and closely contacts therewith toclose the opening 109 a, thereby self-sealing to maintain the pressurein the hydraulic chamber 103 greater than that of the gas chamber 104.

It should be noted that the resonance box 110 of the plug 108 isdesigned such that the volume, the length, and the cross section are notvariable. The volume of the resonance box may be variable while theaccumulator is operated. For example, the resonance box may be dividedinto plural chambers, which may be opened or closed by valves driven byan external system, so that the volume of the resonance box can usuallybe variable. The length of the flow path 109 may be variable, such as inperiscopes, and the length may be driven by an actuator. The openingarea, that is, the cross section of the flow path 109, may be variableby butterfly valves and the like.

The third embodiment of the invention will be explained with referenceto FIGS. 8 to 13 hereinafter.

FIG. 8 is a cross section showing an accumulator of the embodimentaccording to the invention. In the figures, reference numeral 210 is acylindrical shell, and 240 is a metallic bellows (buffer member) whichpartitions the interior of the shell 210 into a hydraulic chamber 211and a gas chamber 212. Reference numeral 250 is a port forming acommunicating path in the hydraulic chamber 211 side, and 260 is a plugretainer to which a plug for sealing the gas chamber 212 is attached.

The shell 210 forms a sealed vessel by joining a bottom shell 220 as amain component and a cap shell 230 of which the axial length is shorterthan that of the bottom shell 220, and the shells 220 and 230 aredivided in the axial direction before the joining. The shells 220 and230 are formed by pressing to a uniform thickness from metals such ascopper, and the bodies thereof extending in the axial direction arejoined to each other by welding.

The bellows 240 consists of a bellows body 241 which is elastic in theaxial direction, a bottom seal 242 fixed at an end of the bellows body241, and a bellows cap 243 fixed at another end of the bellows body 241.The bottom seal 242 and the bellows cap 243 are connected to the bellowsbody 241 by a welding method such as tungsten inert gas welding orplasma welding. In the bellows 240, the bottom seal 242 is fixed to theport 250 by welding, the inner space above the bottom seal 242 forms thehydraulic chamber 211, and the space defined by the bellows 240 and theshell 210 constructs the gas chamber 212. The hydraulic chamber 211 iscommunicated with a hydraulic system (not shown), and an inert gas suchas nitrogen gas is sealed in the gas chamber 212 at a predeterminedpressure. A hydraulic opening 242 a is formed at the center of thebottom seal 242. A self seal 244 made from rubber is adhered to theinner surface of the bellows cap 243. The self seal 244 preventexcessive compression of the bellows body 241 and damage of the bellowscap 243 due thereto.

The port 250 is a cylindrical body consisting of a fitting circumference251 which fits into a through hole 230 a formed at the center of the capshell 230, and a circular step portion 252 extending outward from thefitting circumference 251 and engaging with the inner surface of the capshell 230. A hydraulic path 250 a communicated with the hydraulic systemis formed at the center of the port 250. The port 250 is inserted intothe through hole 230 a from inner side of the cap shell 230, the stepportion 252 is engaged with the inner surface of the cap shell 230, thefitting circumference 251 is fitted into the through hole 230 a, and theport 250 is then welded to the cap shell 230.

The plug retainer 260 is a cylindrical body consisting of a fittingcircumference 261 which fits into a through hole 220 a formed at thecenter of the bottom shell 220, and a circular step portion 262extending outward from the fitting circumference 261 and engaging withthe inner surface of the bottom shell 220. The plug retainer 260 isfixed to the bottom shell 220 by welding. A gas feeding opening 260 a isformed at the center of the plug retainer 260. The gas feeding opening260 a is sealed by screwing or welding a plug (not shown) therein aftera gas is sealed in the gas chamber 212.

A bellows guide 270 is fixed at the circumference of the bellows cap243. The bellows guide 270 is ring-shaped and slideably fitted into theinner surface of the shell 210. Plural through holes 270 a are formed atthe outer circumference of the bellows guide 270 therealong at regularintervals. The bellows guide 270 support the free end of the bellowsbody 241 so that it may not swing in the elastic motion thereof. In theelastic motion of the bellows body 241, the gas in the gas chamber 212passes through the through hole 270 a.

According to the accumulator having the above construction, hydraulicfluid is fed into the hydraulic chamber 211 through the hydraulicopening 242 a of the bottom seal 242 from the hydraulic path 250 a ofthe port 250. When the pressure of the hydraulic fluid in the hydraulicchamber 211 exceeds the pressure in the gas chamber 212, the bellowsbody 241 is extended and the gas in the gas chamber 212 contracts. Whenthe pressure of the hydraulic fluid exceeds the pressure in the gaschamber 212, the bellows body 241 is extended and the gas in the gaschamber 212 contracts. When the pressure of the hydraulic fluid in thehydraulic chamber 211 is lower than the pressure in the gas chamber 212,the bellows body 241 is contracted and the gas in the gas chamber 212expands. For the expansion and contraction of the gas in the gas chamber212, variation of the pressure in the hydraulic fluid in the hydraulicsystem is absorbed, and pulsation of the hydraulic fluid can beinhibited.

The manufacturing process according to the invention will be explainedin order of steps.

(A) Attachment of bellows and port to cap shell

As shown in FIG. 8, the bellows body 241 is fixed to the bottom seal 242by welding such as tungsten inert gas welding or plasma welding. Then,the bottom seal 242 is welded to the port 250. As shown in FIG. 9A, anedge 245 at approximately a right angle before the welding is formed atthe inner surface of the bent portion on the lower surface of the bottomseal. The edge 245 is brought into contact with the welding portion ofthe port 250 and is pushed to the port 250, and these are then welded byelectric resistance welding. This welding is projection welding sincethe edge 245 is a projection, and the edge 245 of the bottom seal 242 ismainly melted and welded.

Then, the cap shell 230 and port 250 are projection welded in the samemanner. As shown in FIG. 10A, in the condition that the port 250 isinserted into the through hole 230 a of the cap shell 230 from theinside thereof before welding, the edge 231 at the inside of the throughhole 230 a (upper side in FIGS. 10A and 10B) is brought into contactwith the fitting circumference 251 of the port 250. In this condition,the edge 231 is pushed to the inside and welded with the fittingcircumference 251 as shown in FIG. 10B. In the welding, the edge 231 ofthe cap shell 230 is mainly melted and welded. Then, as shown in FIG. 8,the bellows cap 243 is welded to the bellows body 241 by welding such astungsten inert gas welding or plasma welding.

(B) Attachment of plug retainer to bottom shell

The plug retainer 260 is projection welded to the bottom shell 220. Asshown in FIG. 11A, when that the plug retainer 260 is inserted into thethrough hole 220 a of the bottom shell 220 from inside thereof beforewelding, the edge 221 at the inside of the through hole 220 a (lowerside in FIGS. 11A and 11B) is brought into contact with the fittingcircumference 261 of plug retainer 260. In this condition, the edge 221is pushed to the inside and is welded with the fitting circumference 261as shown in FIG. 11B. In the welding, the edge 221 of the bottom shell220 is mainly melted and welded.

After the above steps (A) and (B), the bellows 240 and the port 250 isattached to the cap shell 230, and the plug retainer 260 is attached tothe bottom shell 220. Then, the bottom shell 220 and the cap shell 230are joined by projection welding.

(C) Joining bottom shell with cap shell

As shown in FIG. 12, circular circumferences 222 and 232 projectingoutward are formed at the joining portion of the shells 220 and 230 overthe entire circumference. The circular circumferences 222 and 232consist of conical portions 222 a and 232 a projecting in the axialdirection at an angle of 45°, and small circumferential portion 222 band 232 b extending in the axial direction from the front edge of theconical portions 222 a and 232 a. In the shells 220 and 230, circularprotrusions 223 and 233 with triangular cross section tapering towardthe joining portion are formed at the end of the circumferentialportions 222 b and 232 b over the entire circumference.

As shown in FIG. 12, a ring-shaped bellows protector 271 is fixed at theinner surface of the cap shell 230. A groove 271 a is formed at theouter surface of the bellows protector 271 over the entire circumferencethereof. The inner diameter of the bellows protector 271 coincides withthe that of the shell 210. The axial length of the bellows protector 271is designed such that there is a clearance between the bottom shell 220and it before welding the bottom shell 220 with the cap shell 230, andthe clearance disappears after the welding. The bellows protector 271 ismade from an insulating resin or the like so as to provide insulatingproperties from the cap shell 230. Alternatively, the bellows protector271 may be made from a metallic material such as steel, and at least aportion which contacts the bottom shell 220 is coated by an insulatingresin so as to provide insulating properties from the cap shell 230.

In welding the shells 220 and 230, as shown in FIG. 12, circularprotrusions 223 and 233 are brought into contact with each other, andthe circular circumferences 222 and 232 are clamped by a pair ofelectrodes 270A and 270B. The circular protrusions 223 and 233 aremutually tightly thrust by pressing them with the electrodes 270A and270B. Maintaining this condition, the electrodes 270A and 270B areenergized and projection welding is performed. In the welding, thecircular protrusions 223 and 233 are melted and welded. The groove angle(angle θ in FIG. 12) in contacting the circular protrusions 223 and 233with each other is about 90°.

In an electric resistance welding, a spark is often emitted from thejoining portion. The spark does not strikes the bellows body 241 sinceit is shielded by the bellows protector 271. Therefore, damages to thebellows body 241 by the spark can be prevented and the service lifethereof can be ensured. Beads projecting inward and outward are formedin an electric resistance welding. The bead projecting inward isinserted into the groove 271 a of the bellows protector 271. The bottomshell 220 come into proximity with the cap shell 230 in the electricresistance welding. As a result, the bottom shell 220 is brought intocontact with the bellows protector 271 and the clearance disappears.

In the method for welding the bottom shell 220 and the cap shell 230,the welding is performed over the entire circumference by instantaneousprojection welding. Therefore, the time required for welding the shells220 and 230 can be greatly shortened compared to the conventionalwelding method. As a result, mass manufacturing efficiency is improvedand production cost can be reduced. The recess at the inner surface ofthe circular circumferences 222 and 232 is embedded with the bellowsprotector 271, so that the inner surface of the shell 210 can be smooth.When the joining portion of the bottom shell 220 and the cap shell 230is located at the intermediate thereof in the axial direction, thebellows guide 270 slides over the joining portion, and the bellows guide270 is guided by the bellows protector 271 so as to smooth the sliding.

Projection welding is not smoothly performed when the materials havelarge differences in the heat capacities thereof. In the embodiment ofthe invention, the thickness of the bottom shell 220 and the cap shell230 are approximately uniform and the heat capacities thereof areapproximately the same. Therefore, the projection welding is smoothlyperformed and the sealing of the shell 210 is reliable and strong. Formaking the thickness of the bottom shell 220 and the cap shellapproximately uniform, press forming is preferably performed withoutmachining and forging. This forming method decreases manufacturing cost.

The accumulator is a type in which the interior of the bellows 240 formsthe hydraulic chamber 211. The manufacturing method in the invention canbe applied to the accumulator in FIG. 13 in which the interior of thebellows 240 forms the gas chamber 212. In the figure, the same numeralsas in FIG. 8 are put on the same elements as in FIG. 8. In theaccumulator, the bottom seal 242 of the bellows 240 is welded to theplug retainer 260, and the gas feeding opening 242 b is formed in thebottom seal 242. The self seal 2444 is adhered to the outer surface ofthe bellows cap 243. The inner space of the bellows 240 forms the gaschamber 212, and the space defined by the bellows 240 and the shell 210forms hydraulic chamber 211. The procedure of assembling the accumulatoris the same as for the above embodiment, except that the bottom seal 242is welded to the plug retainer 260 instead of the port 250, and theaccumulator can be manufactured with the same welding method as in theabove embodiment.

It should be noted that although the accumulator in the embodiment usesthe metallic bellows 240 as buffer members for partitioning the interiorof the shell 210 into the hydraulic chamber and the gas chamber 212, thebellows 240 may be made from materials other than metal. The buffermember is not limited to bellows, and pistons, diaphragms, and balloonsmay be used. Although the hydraulic fluid goes in and out the hydraulicpath 250 a in the embodiment, the invention may be applied to theinline-type accumulator in which an inlet and an outlet to the hydraulicchamber 211 may be individually provided and the hydraulic fluid is fedalong the axial direction.

What is claimed is:
 1. A manufacturing process for an accumulator, theprocess comprising: assembling a buffer member into a cylindrical shellso as to partition a interior of the shell into a gas chamber and ahydraulic chamber; and closing the shell; wherein the shell comprisesshell portions divided in the direction of an axis of the shell; forminga circular circumferential portion projecting outward over the entirecircumference of each joining portion of the shell portions; forming acircular protrusion projecting in a joining direction at the circularcircumferential portion of at least one of the shell portions;contacting the circular protrusions with each other, alternatively thecircular protrusion is brought into contact with the circularcircumferential portion of another shell portion; clamping and pressingthe circumferential portions by a pair of electrodes; and energizing theelectrodes so as to join the joining portions by electric resistancewelding.
 2. A manufacturing process for an accumulator according toclaim 1, wherein a protector is provided at an inner surface of thecircular circumference of one of the shell portions, the protectoravoids a spark occurred in the electric resistance welding from emittinginto the inside of the shell portion.
 3. A manufacturing process for anaccumulator according to claim 2, wherein a recess is provided at aninner surface of the circular circumferences, the protector is containedinto the recess.
 4. A manufacturing process for an accumulator accordingto claim 3, wherein the protector has an inner diameter, which coincideswith an inner diameter of the shell portions.